USB Battery Pack Conversions (mAh to Wh)

I know I haven’t posted in a while – but I’ve had lots going on…. Then my first post back is a techie one, I apologise (well for some of you I do anyway!).

I want a backup battery for my Motorola Droid/Milestone, I’ve already got one for my iPod Touch and it’s invaluable – but I figure if I get one with a straight USB out it can charge almost any of my devices with the right cable! This makes far more sense to me, so today I went hunting. Would you believe how hard these things are to find?

I’ve been offered rechargable batteries, battery chargers, spare batteries for the phone all sorts of things – none of which were quite what I wanted, I think this boils down to not knowing what to search for – generally most of what I found was listed as:

“Rechargeable Backup Battery Pack USB”

I’ve found to, one by an unknown manufacturer (PINES) and one by Kensington, so I start doing a little comparison on them, the features I need are:

• Small
• Light
• Capacity

Shouldn’t be that hard really, nobody can play with dimensions or weight (although there will probably be trade-offs for capacity!) but they’re pretty much standard:

Almost sold then, Kensington tell me lots of information about their pack and it’s 7wH capacity, compared to Pines 4800mAh?
Wait a minute…. Are we comparing apples with apples and beans with beans here?

What are wH and how do we convert them to mAH?

I found a useful link on camerahacker with conversions, so let’s crunch the numbers.. It’s probably easier to convert the Pines product so it is also in wH

watt = amp * volt

So,

Wh = 4.8Ah * 5.5VDC (Note we've converted the mAh to Ah by dividing it by 1000)
Wh = 26.4

Compared to the Kensington device – that’s huge! 26.4Wh vs 7Wh (and it’s cheaper!)…
Have I got something wrong here?